Processes for producing olefins by the dehydrogenation of saturated hydrocarbons are well known. A typical dehydrogenation process mixes the feed hydrocarbons with hydrogen and heats the resulting admixture by indirect heat exchange with the effluent from the dehydrogenation zone. Following heating, the feed mixture passes through a heater to further increase the temperature of the feed components before it enters the dehydrogenation zone where it is contacted with the dehydrogenation catalyst. The catalyst zone may be operated with a fixed bed, a fluidized bed, or a movable bed of catalyst particles. After heat exchange with the feed, the dehydrogenation zone effluent passes to product separation facilities. The product separation facilities will typically produce a gas stream, made up primarily of hydrogen, a first product stream that includes the desired olefin products, and a second potential product stream comprising light hydrocarbons. The light hydrocarbon stream typically has fewer carbon atoms per molecule than the desired olefin product. Light hydrocarbons are generally removed from the product stream in order to reduce flow volume, operating pressures, and undesirable side reactions in downstream process units that receive the olefin product. A portion of the hydrogen stream is typically recycled to the dehydrogenation zone to provide hydrogen for the combined feed stream. The product stream usually contains unconverted dehydrogenatable feed hydrocarbons in addition to the product olefin. These unconverted hydrocarbons may be withdrawn in the separation facilities for recycle to the dehydrogenation zone or passed together with the product olefins to an etherification zone for conversion of the product olefins to ethers. Etherification processes are currently in great demands for making high octane compounds which are used as blending components in lead-free gasoline. These etherification processes will usually produce ethers by combination of an isoolefin with a monohydroxy alcohol. The etherification process can also be used as a means to produce pure isoolefins by cracking of the product ether. For instance, pure isobutylene can be obtained for the manufacture of polyisobutylenes and tert-butyl-phenol by cracking methyl tertiary butyl ether (MTBE). The production of MTBE has emerged as a predominant etherification process which uses C.sub.4 isoolefins as the feedstock. A detailed description of processes, including catalyst, processing conditions, and product recovery, for the production of MTBE from isobutylene and methanol are provided in U.S. Pat. Nos. 2,720,547 and 4,219,678 and in an article at page 35 of the June 25, 1979 edition of Chemical and Engineering News. The preferred process is described in a paper presented at The American Institute of Chemical Engineers, 85th National Meeting on June 4-8, 1978, by F. Obenaus et al. Another etherification process of current interest is the production of tertiary amyl ether (TAME) by reacting C.sub.5 isoolefins with methanol.
Due to the limited availability of olefins for etherification, it has become common practice to produce them by the dehydrogenation of isoparaffins and to pass the dehydrogenation effluent to an etherification process. General representations of flow schemes where a dehydrogenation zone effluent passes to an etherification zone are shown in U.S. Pat. Nos. 4,118,425 and 4,465,870. More complete representations of a flow arrangement where the dehydrogenation zone effluent passes to an etherification zone are given in U.S. Pat. No. 4,329,516 and at page 91 of the October, 1980 edition of Hydrocarbon Processing. The latter two references depict the typical gas compressin and separation steps that are used to remove hydrogen and light ends from the dehydrogenation zone effluent before it passes to the etherification zone. A typical effluent from an etherification zone includes an ether product, unreacted alcohol, and unreacted hydrocarbon. These effluent components enter separation facilities that yield the ether product, alcohol for recycling to the etherification zone, hydrocarbons for further processing into dehydrogenation. This recycle stream of C.sub.4 or C.sub.5 isoparaffins, prior to recycling to the dehydrogenation zone, is usually treated to recover methanol and remove other oxygenates which are harmful to the dehydrogenation catalyst.
As evidenced by the foregoing references, the light materials that are present with the effluent from the dehydrogenation zone are viewed as undesirable and have been removed ahead of the etherification processes. These undesirable light materials, in the case of C.sub.4 olefin conversion to produce butyl ethers, will normally include hydrogen, methane, and ethane. In the case of C.sub.5 olefin conversion in the production of aryl ethers, the undesirable materials can include C.sub.4 hydrocarbons.
It is a broad object of this invention to improve the arrangement and operation of an etherification process that receives the dehydrogenating feed stream of dehydrogenated hydrocarbons.
A more specific object of this invention is to reduce the capital and utility cost associated with the separation and recycle of components from the effluents of the combined processes for dehydrogenating hydrocarbons and the production of ethers.
Another object of this invention is to simplify the separation facilities in a combined process for the dehydrogenation of dehydrogenatable hydrocarbons and the etherification of the dehydrogenated hydrocarbons.